I. Field of the Invention
The present invention relates to communications systems. More particularly, the present invention relates to a novel and improved method for hard handoff between different wireless communication systems.
II. Description of the Related Art
In a code division multiple access (CDMA) spread spectrum communication system, a common frequency band is used for communication with all base stations within that system. An example of such a system is described in the TIA/EIA Interim Standard IS-95-A entitled xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, incorporated herein by reference. The generation and receipt of CDMA signals is disclosed in U.S. Pat. No. 4,401,307 entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d and in U.S. Pat. No. 5,103,459 entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d both of which are assigned to the assignees of the present invention and incorporated herein by reference.
Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high rate pseudonoise (PN) code. A PN code is used to modulate signals transmitted from the base stations and the remote stations. Signals from different base stations can be separately received at the receiving station by discrimination of the unique time offset that is introduced in the PN codes assigned to each base station. The high rate PN modulation also allows the receiving station to receive a signal from a single transmission station where the signal has traveled over distinct propagation paths. Demodulation of multiple signals is disclosed in U.S. Pat. No. 5,490,165 entitled xe2x80x9cDEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALSxe2x80x9d and in U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both of which are assigned to the assignee of the present invention and incorporated herein by reference.
The common frequency band allows simultaneous communication between a remote station and more than one base station, a condition known as soft handoff disclosed in U.S. Pat. No. 5,101,501 entitled xe2x80x9cSOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d and U.S. Pat. No. 5,267,261 entitled xe2x80x9cMOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated herein by reference. Similarly, a remote station can be simultaneously communicating with two sectors of the same base station, known as softer handoff as disclosed in U.S. Pat. No. 5,625,876 entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATIONxe2x80x9d, issued Apr. 29, 1997, assigned to the assignee of the present invention and incorporated herein by reference. Handoffs are described as soft and softer because they make the new connection before breaking the existing one.
If a mobile station travels outside the boundary of the system with which it is currently communicating, it is desirable to maintain the communication link by transferring the call to a neighboring system, if one exists. The neighboring system may use any wireless technology, examples of which are CDMA, NAMPS, AMPS, TDMA or FDMA. If the neighboring system uses CDMA on the same frequency band as the current system, an inter-system soft handoff can be performed. In situations where inter-system soft handoff is not available, the communication link is transferred through a hard handoff where the current connection is broken before a new one is made. Examples of hard handoffs are those from a CDMA system to a system employing an alternate technology or a call transferred between two CDMA systems which use different frequency bands (inter-frequency hard handoff).
Inter-frequency hard handoffs can also occur within a CDMA system. For example, a region of high demand such as a downtown area may require a greater number of frequencies to service demand than the suburban region surrounding it. It may not be cost effective to deploy all available frequencies throughout the system. A call originating on a frequency deployed only in the high congestion area must be handed off as the user travels to a less congested area. Another example is that of a microwave or other service operating on a frequency within the system""s boundaries. As users travel into an area suffering from interference from the other service, their call may need to be handed off to a different frequency.
Handoffs can be initiated using a variety of techniques. Handoff techniques, including those using signal quality measurements to initiate handoff, are found in U.S. Pat. No. 5,697,055 entitled xe2x80x9cMETHOD AND APPARATUS FOR HANDOFF BETWEEN DIFFERENT CELLULAR COMMUNICATIONS SYSTEMSxe2x80x9d, issued Dec. 9, 1997, assigned to the assignee of the present invention and incorporated herein by reference. Further disclosure on handoffs, including measurement of roundtrip signal delay to initiate handoff, is disclosed in U.S. Pat. No. 5,848,063 entitled xe2x80x9cMETHOD AND APPARATUS FOR HARD HANDOFF IN A CDMA SYSTEMxe2x80x9d, issued Dec. 8, 1998, assigned to the assignee of the present invention and incorporated herein by reference. Handoffs from CDMA systems to alternate technology systems are disclosed in U.S. Pat. No. 5,594,718 (""718 patent) entitled xe2x80x9cMETHOD AND APPARATUS FOR MOBILE UNIT ASSISTED CDMA TO ALTERNATIVE SYSTEM HARD HANDOFFxe2x80x9d, issued Jan. 14, 1997, assigned to the assignee of the present invention and incorporated herein by reference. In the ""708 patent, pilot beacons are placed at the boundaries of the system. When a mobile station reports these pilots to the base station, the base station knows that the mobile station is approaching the boundary.
When a system has determined that a call should be transferred to another system via hard handoff, a message is sent to the mobile station directing it to do so along with parameters that enable the mobile station to connect with the destination system. The system has only estimates of the mobile station""s actual location and environment, so the parameters sent to the mobile station are not guaranteed to be accurate. For example, with beacon aided handoff, the measurement of the pilot beacon""s signal strength can be a valid criteria for triggering the handoff. However, the appropriate cell or cells in the destination system which are to be assigned to the mobile station (known as the Active Set) are not necessarily known. Moreover, including all the likely possibilities may exceed the maximum allowable in the Active Set.
In order for the mobile station to communicate with the destination system, it must lose contact with the old system. If the parameters given to the mobile station are not valid for any reason, i.e. changes in the mobile station""s environment or lack of precise location information at the base station, the new communication link will not be formed, and the call may be dropped. After an unsuccessful handoff attempt, the mobile station can revert back to the previous system if it is still possible to do so. With no further information and no significant change in the mobile station""s environment, repeated attempts to hand off will also fail. Thus, there is a need felt in the art for a method for performing additional hard handoff attempts with greater probability of success.
The purpose of this invention is to reduce the probability of dropped calls during inter-system hard handoff. In the event that a hard handoff attempt is unsuccessful, the mobile station will return to the original system with information which the communication system of the present invention uses to assist in the performance of future handoff attempts.
Prior to handoff, the original base station will have a rough estimate of the most likely base stations of a destination system to provide service to a mobile station as it travels into the destination system. In the exemplary embodiment, a message will be sent from the base station to the mobile station containing this list of neighboring base stations in the destination system, a minimum total received power threshold, and a minimum pilot energy threshold. When the base station in the original system has determined that a hard handoff is appropriate, it signals the neighboring base stations in the destination system to begin transmitting forward link traffic to the mobile station entering the system. A first hard handoff is attempted after a message is received by the mobile station from the base station initiating the inter-system hard handoff. The mobile station switches to the frequency of the destination system and attempts to acquire the base stations of the destination system in accordance with the acquisition parameters provided (i.e. the pilot PN offsets). If the minimum pilot energy threshold is exceeded, the handoff is deemed to be successful and the mobile station remains on the destination system.
If the minimum pilot energy threshold is not exceeded, recovery techniques begin. The mobile station measures the total in-band energy of the destination system and compares that to the total received power threshold. If the minimum total received power threshold is not exceeded, the handoff is abandoned immediately. The mobile station returns to the original system and reports that no significant power was detected at the new frequency. If the minimum total received power is exceeded, it is likely that the destination system is available, but that the neighboring base stations provided by the original system (referred to as the new Active Set) is not acceptable for communication. The mobile station then performs a search to locate viable pilot signals in the destination system. In general, a list of offsets to search provided to the mobile station will be sufficient to locate available pilots, although other search algorithms can be employed. Upon completion of the search, the mobile station returns to the original system and reports the failure and any pilot signals found in the search which exceeded a third threshold.
If no significant received power was detected or there were no pilots found in the search, the system controller can opt to delay a second attempt at handoff in hopes of a beneficial change in the mobile station""s environment. In the alternative, the mobile station can abandon the hard handoff attempt altogether, which would likely result in the eventual dropping of the call. However, in those cases where the destination system is present, the system controller can update the Active Set based on the returned search information, and the destination system can modify the base stations transmitting to the mobile station accordingly. Then a second hard handoff attempt message can be sent to the mobile station. Unless the environment has changed, this second attempt is likely to be successful.